glibc: 2.25-49 -> 2.26-75

Security: the NEWS claims a couple more CVEs are fixed than what we
patched, though perhaps nothing critical.

I personally don't find DNS fragmentation attacks that interesting
anymore, as it's just about weaker improvements for cases that choose
not to use DNSSEC.

Largest expected caveat: upstream bumped the minimal supportable kernel
to 3.2.0.  That's the oldest kernel still supported upstream, released
in Jan 2012, but most notably RHEL 6 and derivates still use a heavily
patched 2.6.32 kernel and those systems are still supported and in use
(production support is scheduled to last till the end of 2020!).